1. Technical Field
The present invention relates to a vibrating reed such as, for example, a piezoelectric vibrating reed made of a piezoelectric material, a vibrator, an oscillator, and an electronic device using these components.
2. Related Art
Piezoelectric devices have been widely used for communication equipment such as mobile phones, information equipment such as computers or IC cards, and other various electronic devices. Along with the miniaturization and higher performance of the electronic devices, high quality and high stability, together with its miniaturization, are recently demanded for the piezoelectric devices.
It is well known in a piezoelectric vibrating reed of the bending vibration mode that reducing the length of a vibrating arm for achieving its miniaturization increases frequency. Therefore, a mass is added to the tip portion of the vibrating arm, and due to the mass effect, the length of the vibrating arm is reduced while avoiding an increase in frequency, thereby miniaturizing the piezoelectric vibrating reed (refer to JP-UM-B-51-10755 and JP-A-2004-282230, for example). In many cases, a mass is added to the tip portion of the vibrating arm by forming the width of the tip portion larger than that of the base end side.
Moreover, for decreasing the CI value in a tuning-fork type piezoelectric vibrating reed, a structure has been widely adopted in which a groove portion in the longitudinal direction is formed at least one of both main surfaces of a vibrating arm, and an excitation electrode is deposited on the inner surface of the groove portion (refer to JP-A-2004-282230, for example). In such a vibrating arm, an electric field is generated between an excitation electrode of its side surface and the excitation electrode within the groove portion so as to widely distribute in a cross-section of the vibrating arm, and an electric field efficiency is greatly improved. Therefore, vibration loss is small even when the vibrating reed is miniaturized, and the CI value can be suppressed to a low level.
Further, a tuning-fork type piezoelectric vibrating reed having a structure in which a groove portion of a vibrating arm is extended to positions entering a weight portion at a tip of the vibrating arm and a base has been proposed (refer to JP-A-2005-5896, for example). In the vibrating arm having such a groove structure, an arm length necessary for obtaining the basic vibration mode of the vibrating arm can be assured, and at each of coupling portions of the vibrating arm with the weight portion and the base, concentration of stress on the groove portion due to the vibration of the vibrating arm can be eliminated to thereby obtain stable vibrations.
In JP-A-2005-5896, a vibrating arm in which a ridge portion is disposed on the base side of the weight portion for decreasing the CI value is disclosed. The vibrating arm will be described in detail with reference to the drawings. In FIGS. 6A to 6C, a tuning-fork type piezoelectric vibrating reed 1 disclosed in JP-A-2005-5896 has a base 2 and a pair of vibrating arms 3 extended in parallel from one end portion of the base 2. Each of the vibrating arms 3 has on the tip side of the vibrating arm 3 a weight portion 5 having a larger width than the vibrating arm 3 on the base 2 side, a first bottomed groove portion 7 formed along the long-side direction of the vibrating arm 3, a first excitation electrode 9 formed on each side surface of the first groove portion 7, and a second excitation electrode 10 formed on each side surface of the vibrating arm 3. On the base 2 side of the weight portion 5, a second groove portion 8 is disposed as a through hole. The second groove portion 8 has as one of side surfaces a side surface which is continuous from the side surface of the vibrating arm 3 and forms the same surface as that of the vibrating arm, and the one side surface is formed in parallel with the side surface of the first groove portion 7. With the second groove portion 8, a ridge portion (protruding portion) 6 is formed at both ends of the weight portion 5 on the base 2 side. Due to the ridge portion 6, in the vibrating arm 3, the arm length as the vibration secondary moment can be reduced without changing the mass of the weight portion 5, whereby the stability of vibration can be enhanced, and the CI value can be suppressed to a low level.
However, in the tuning-fork type crystal vibrating reed 1 disclosed in JP-A-2005-5896, since the second groove portion 8, which has as one of side surfaces the extending portion of the side surface of the vibrating arm 3, of the weight portion 5 is a through hole penetrating through both main surfaces of the weight portion 5, there is a risk of causing a disadvantage because each of the ridge portions 6 formed at both ends of the weight portion 5 on the base 2 side serves as a weight. That is, the present inventor has found that when the vibrating arm 3 repeats bending vibrations in directions indicated by the arrows in the drawing, the weight portion 5 exhibits a larger displacement than the vibrating arm 3 at the peak of the amplitude of vibration of the vibrating arm 3 and is brought into a vibration mode close to the higher-order mode, whereby frequency is increased to cause unstable vibrations and cause a risk of increasing the CI value.